(i) Field of the Invention
The present invention relates to an ink composition for printing which can be used to cover circuit boards, and particularly it relates to an ink composition for printing which has improved printability, adhesive properties, flexibility and heat resistance.
(ii) Description of the Related Art
Heretofore, cover films have been used as covering base materials for printing circuits. With regard to the usage of these cover films, either surface of each polyimide film or polyester film is first coated with an adhesive, and the film is punched at the position thereof corresponding to a terminal connection site in a manner such as punching. Afterward, this film is positioned on a circuit board by hand and then stuck thereon at a high temperature under a high pressure by means of a device such as a heat plate press.
This process is excellent in flexibility and is advantageous from the viewpoint of a circuit protection, but it has some problems. For example, when the terminal of a distribution frame is led, the cover film is required to be punched previously, thereby perforating a small hole through the cover film. Therefore, when the circuit is complex, the positioning of the film to the distribution frame is difficult. In addition, facilities such as the hot plate press are expensive. Furthermore, since adhesive is used, when punching is missed this tends to occur in the punching step, which increases the cost. In addition, the adhesive is likely to ooze when the film is pressed.
Moreover, even if the physical properties of the film itself are good, the performance of the whole cover film depends unavoidably upon the level of the adhesive, so long as adhesive is used, because the quality itself of the adhesive is still unsatisfactory, and any adhesive so good as to be comparable to the base film in points of heat resistance and electrical characteristics has not been found so far.
A method (cover lay ink) for forming insulating protective films on print-circuit boards by a printing process has been developed. However, the cover layer ink agent presently being used applies solder resists which has been heretofore used for rigid boards. This cover layer ink is poor in flexibility, heat resistance, electrical properties and adhesive properties to the base material, and when the cover layer ink is applied to flexible print circuit boards (hereinafter abbreviated to "FPC"), there is the problem that curling takes place owing to a difference between thermal expansion coefficients of the cover layer ink and the boards, and owing to curing shrinkage. In addition, the reaction rate of the cover layer ink with a resin is slow, and therefore most of the cover layer inks are of a twopack type. For this reason, the handling of the cover layer ink is troublesome, and after the two liquids are mixed, a pot life is short and a storage stability is poor.
For example, Japanese Patent Application Laid-open No. 55-145717 discloses a composition comprising an epoxy acrylate resin and a melamine resin, but this composition has the drawback that the flexibility is poor.
Furthermore, Japanese Patent Publication Nos. 50-4395 and 53-10636 disclose acrylic resin compositions such as sulmephomethylene acrylate and ethylene phosphate acrylate, and all of them are excellent in flexibility but poor in heat resistance.
Japanese Patent Application Laid-open No. 63-221172 discloses a resin composition mainly comprising a polyaminobismaleimide and an epoxy resin, and Japanese Patent Application Laid-open No. 1-121364 discloses a composition containing a polyimide as the main component, and Japanese Patent Application Laid-open No. 1-256515 discloses a composition mainly comprising a polyparabanic acid and an epoxy resin. These compositions noticeably shrink at the time of curing and there is a large difference between the thermal expansion coefficients of the cured resin composition and the board, and for these reasons, the FPC boards on which the cover layer coating is formed curl, noticeably.
In Japanese Patent Application Laid-open No. 2-283762, a composition mainly comprising a polyimide and a low-elastic resin is disclosed, and the FPC board scarcely curls on which the cover layer film of such a composition is formed. In this composition, however, there is used the polyimide resin which scarcely has such physical properties as to withstand its practice, if it does not have a substantially high molecular weight, and therefore its solid content concentration inevitably remains at a low level. Accordingly, in order to obtain the cover layer film having a practically necessary thickness, it is essential to carry out the printing many times, which makes production efficiency low. This is a serious bottleneck in the process. Furthermore, for the curing, the conditions of extremely high temperature are required, and so a conductive foil of the board is likely to be oxidized in the curing step. For the achievement of the curing at a high temperature, a specific curing furnace and the like are necessary, so that the cost increases.